Plain-woven fabrics are widely utilized as structural materials for many applications including inflated structures, fabric roofing, and large canopies, among others. In these capacities, the materials are primarily affected by variations in weather, i.e., temperature changes, precipitation, and wind, though stress due to weight and pressure is also a concern. Therefore, while such structures are designed to be lightweight and pliable, they must also exhibit superior strength and durability.
These materials may also find application in lighter-than-air vehicles, such as aerostats, balloons, and airships, including blimps and dirigibles, which are used in many different applications, such as near large sporting, entertainment or cultural events, or in large metropolitan areas to provide advertising or to provide high level coverage of the events. Lighter-than-air vehicles are also used in high altitude applications, for the purpose of weather monitoring and/or military surveillance. In such instances, the higher a vehicle can operate translates into an increased amount of area that can be viewed for surveillance purposes and/or weather monitoring. Additionally, lighter-than-air vehicles that possess the ability to operate at altitudes above 50,000 feet are not a hazard to commercial air traffic, are more difficult to detect and/or destroy, can be used for the surveillance of wide areas, and thus can provide a strategic and/or economic, as well as providing a means to relay communications.
Of particular interest herein are structural materials for use as the hull material in lighter-than-air vehicles, though the apparatus and method of using the same disclosed herein will find application in testing materials for any of the foregoing uses. For example, typical high altitude lighter-than-air vehicles are made from flexible, fabric laminates including lightweight materials that withstand a wide range of temperature variation and daily expansion and contraction due to such temperature variations, ozone degradation, and exposure to ultraviolet light. Materials employed for more conventional uses may experience these same conditions, though generally to a lesser extreme.
Many uses of these materials result in localized stresses in addition to overall stress and strain. For example, if the material bears logo or identification lettering, as in the case of advertising balloons or dirigibles, the logo or lettering may generate localized heat accumulation. Other potential localized stress areas are seams where panels of flexible, fabric laminate are joined together using structural seam tape to form larger structures. The area along the edge of the seams is subject to greater stress, due to the transition in stiffness of the fabric laminate to the seam area, resulting in increased potential for material failure.
In light of the environmental extremes and other stresses experienced by materials used for any of the foregoing applications, and particularly those experienced by materials used for lighter-than-air vehicle application, the materials of choice for such applications are typically high strength materials. For example, U.S. Pat. No. 6,074,722 to Cuccias teaches a fabric laminate made of a layer of polyurethane resin used to bond layers of high strength liquid crystal thermotropic (melt spun) polyester (VECTRAN®), aromatic polyaramide (KEVLAR®), or polyester (DACRON®) fiber woven yarn to a polyvinyl fluoride (TEDLAR® or MYLAR®) layer, and having an outer layer of a material that is resistant to degradation by ultra violet radiation. U.S. Pat. No. 6,979,479 teaches a laminate of a liquid crystal polymer fiber yarn layer (VECTRAN®) as an interior surface, an adhesive layer, a polyimide layer, and a polyvinylidene fluoride (PVDF) layer which forms the exterior surface.
In these materials, the various layers function as a gas barrier, to retain helium or hydrogen, and/or to provide protection from degradation caused by, for example, ozone or ultraviolet radiation. The flexible, fabric laminate may further include a thin metallic coating to provide a means for passive thermal management, reduce helium permeation, minimize the affects of lightening strikes, and provide a means for uniform static electricity distribution over the hull surface.
As noted above, the flexible, fabric laminate materials experience a variety of extreme environmental parameters. In addition, stress and strain is compounded by the need to use materials that minimize the weight of the vehicle or device. For example, a reduction in the quantity of laminating adhesive, opening the fabric weave to leave more space between fibers or yarns and using the smaller, lower denier yarn can help to reduce the weight but may also reduce the strength and increase local stress concentrations in a fabric laminate. Clearly, there is a fine balance between the necessity to use a lightweight material, yet use a material that can withstand extreme operating conditions.
In light of the foregoing, it is imperative that the properties of a flexible, fabric laminate and its constituent materials be known in as detailed a manner as possible in order to predict potential use limitations, up to the point of material failure. Knowledge of the strength and property limitations of flexible, fabric laminate materials, and particularly of lightweight, high strength fabric laminate materials, will allow for use of the materials within the confines of certain system designs.
Known machines and apparatuses that are used to test material strengths and property limitations of the type discussed hereinabove are limited to devices capable only of creating uniaxial and/or symmetrical biaxial stress. Typically, a tensile test machine is used to generate uniaxial forces to evaluate the strength and performance of materials. However, for inflated structures, such as an airship, aerostat, or blimp, the stresses in the hull material due to the inflation pressure are biaxial and unlikely to be equal. For example, the stress in the hoop or circumferential direction is two times the stress in the longitudinal or axial direction. To characterize and evaluate the actual performance of airship hull fabric laminates, it is necessary to test them while under simultaneous, unequal, independent, biaxial stress. If the lighter-than-air vehicle is spherical, such as a balloon, the stresses are generally equal in all directions.
Certain machines are capable of testing in-plane shear strength as well; however, machines with this capability do not create independent biaxial stress. While machines with the ability to apply symmetric biaxial force can simulate operational stresses to a certain degree, they fail to provide a means to accurately test a fabric laminate under more realistic applied stress conditions that the material will experience in use, particularly, the application of sheer stress simultaneously with independently varied biaxial stresses.
As can be seen from the foregoing, what is needed is an apparatus capable of generating biaxial loads, independently of each other, to assess biaxial stress and strain, and to further test and evaluate in-plane sheer. In addition, such an apparatus that also assesses gas permeability, gas barrier film delamination and other such parameters while simultaneously subjected to biaxial loads and in-plane shear is needed to more accurately predict the performance of flexible, fabric laminate materials under extreme conditions.